The present disclosure relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such physiological characteristics. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
A physiological characteristic that may provide information about the clinical condition of a patient is the total concentration of hemoglobin in blood (HbT) or the hematocrit (Hct), which relates to the fraction or percentage of red cells in whole blood. The hematocrit is the fraction of the total blood volume occupied by the red blood cells, and hemoglobin is the principal active constituent of red blood cells. Approximately 34% of the red cell volume is occupied by hemoglobin.
Measurements of a patient's hematocrit (Hct) levels may involve an invasive technique. For example, a healthcare provider may puncture the skin, draw blood from a vein or capillary into a small-diameter tube, and measure the solid (packed-cell) fraction that remains after centrifugation of the blood. Similarly, measurement of HbT in often may involve a healthcare worker drawing a blood sample, which is then subjected to a chemical or mechanical process to lyse the red cells and release the liquid hemoglobin. After transferring the hemoglobin to a cuvette, its concentration may be measured either by direct spectrophotometry or by colorimetry, following the addition of a chemical reagent. Both of these techniques are relatively labor-intensive, as they involve the participation of skilled healthcare workers in drawing the blood and skilled laboratory workers to perform the subsequent analysis.
Certain noninvasive methods for measurement of hematocrit or total hemoglobin concentration involve spectrophotometric measurement of blood in intact skin. The method is based in part on the measurement of the ratios of the pulsatile (AC) and non-pulsatile (DC) components of the light transmitted through a blood-perfused tissue within two spectral bands in which the molar extinction coefficients of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) are nearly the same. In one of the wavelength bands, the absorption of hemoglobin is the dominant contributor to the attenuation of light in blood; in the other band, the scattering and absorption of surrounding tissue constituents dominates. Therefore, the scattering and absorption of surrounding tissue constituents serves as a measure of the probed volume in the tissue bed.
In spite of the use of noninvasive techniques, measuring the absolute concentration of hemoglobin in blood accurately and reliably remains difficult in practice. Areas of low perfusion may generate measurement signals that are overwhelmed by the scattering and absorption of surrounding tissues. In addition, variable pulsatile changes in blood volume may introduce measurement variability.